Metal–organic frameworks and their composites for fuel and chemical production via CO2 conversion and water splitting

Increase in the global energy demand has been leading to major energy crises in recent years. The use of excess fossil fuels for energy production is causing severe global warming, as well as energy shortage. To overcome the global energy crisis, the design of various chemical structures as efficient models for the generation of renewable energy fuels is very much crucial, and will limit the use of fossil fuels. Current challenges involve the design of Metal–Organic Framework (MOF) materials for this purpose to diminish the energy shortage. The large surface area, tunable pore environment, unique structural property and semiconducting nature of the highly porous MOF materials enhance their potential applications towards the production of enhanced energy fuels. This review is focused on the architecture of MOFs and their composites for fuels and essential chemicals production like hydrogen, methane, ethanol, methanol, acetic acid, and carbon monoxide, which can be used as renewable fuel energy sources to limit the use of fossil fuels, thereby reducing global warming.


Introduction
Considering the current energy demand, fossil fuels are used on a large scale mostly for electricity production to fulll the global energy requirements. The burning of fossil fuels releases large amounts of greenhouse gases including carbon dioxide (CO 2 ) into the atmosphere, which causes global warming. CO 2 absorbs energy from infrared radiation for a longer time and releases it back into the air, thereby increasing the global temperature. As a consequence, various disasters are occurring, such as the melting of glaciers, oods, and the rise in sea levels, making the world an unsuitable place to survive for all living beings, as well as to the environment. The use of excess fossil fuels to accomplish energy requirements leads to the energy crises. It awakens researchers to come up with alternate energy sources to satisfy the energy demand. Therefore, challenges arise in the design of different models to produce efficient energy sources with no environmental drawbacks. Metalorganic framework (MOF)-based structures can be considered an effective model to produce renewable energy sources, photocatalytically, electrocatalytically and photoelectrocatalytically. There are some reports on MOF-based materials that mention their potential applications in fuel cells, lithium-ion batteries, gas tanks, solar cells and supercapacitors. [1][2][3] There are many reports on energy storage and conversion applications using MOF-based materials. [4][5][6] Scientists are trying to solve the energy and environmental issues in many ways. The structural diversity, uniform porosity, host-guest interactive property, tunability, functionalization and semiconducting behavior of MOFs stimulate the researchers to use MOF-based materials for energy storage and conversion applications. Furthermore, these unique properties of MOFs make them superior to other inorganic porous materials. The presence of inorganic metal ions and photoactive-functionalized organic ligands in the MOFs framework enhances their potential to act as catalytically active surface sites for promoting chemical reactions in the presence of light sources to boost energy applications. 1 The integration of various metals, metal oxides, phosphides, sulphides, quantum dots and carbon based materials is now becoming a very challenging topic in the architecture of metal-organic framework composites for various applications. 2,7,8 The incorporation of electron efficient guest molecules into MOFs has a greater impact on many kinds of applications. The excellent gas adsorption and storage ability of MOFs signify their potential uses in fuel cells, vehicle gas reservoirs and others. 1 The development in nanotechnology and proper characterization equipment helps to design and characterize new materials in a proper way to enhance energy resources.
This review is based on the study of previous reports in the design of various MOFs and their composites for the development of renewable energy resources to boost the global economy. Different MOF-based materials with different synthetic methodology and advantages/disadvantages for energy related applications like fuel production have been discussed in this article. Specically, sources of fuel energy from the system. In most cases, the incorporation of different functional materials leads to enhanced activities. Therefore, the syntheses of different MOFs composites have become a topic of interest nowadays. Considering the major energy crisis and environmental issues, scientists have shown great interest in working in energy applications using MOF-based materials owing to the budding research in fuel cells, supercapacitors, photocatalytic hydrogen evolution and CO 2 reduction into useful chemicals and fuels in recent years (Fig. 2). 2,5 There are several methods for synthesizing MOFs and their composites, such as solvothermal, hydrothermal, mechanochemical, microwave synthesis, son-chemical, microemulsion, diffusion, template strategy and electrochemical methods, which have been reported so far. 23 Among these, solvothermal and hydrothermal methods have been considered the most prevailing methods for the synthesis of MOF-based materials. Other synthetic methods are also considered as effective ones, which provide nanoscale controllable MOFs crystals in short reaction times. The synthesis of MOF-based materials depends on several parameters like the temperature, pressure, reaction time, metal nodes, ligands, presence of counterions and reaction kinetics that leads to nucleation and crystal growth. 24 Solvent selection is one of the most important things for any kind of reaction, as it is a key to determine the activation energy and reaction thermodynamics. In addition, the single crystal growth in MOFs is of utmost importance as many of us face difficulties to do so. Furthermore, single crystals are highly ordered structures with unique thermal, optical, mechanical, electrical and other superior properties than polycrystalline materials. Owing to the excellent optical and electrical properties of single crystals, they are utilized in various important applications such as wireless and satellite communication, light-emitting diodes (LEDs), electronics, photodetectors, and wide band-gap devices. With the developing technology, demands for single crystals are growing accordingly. The growth of single crystals with high quality is a very challenging topic since the last decade. From the literature, it has been found that there are three general approaches for the growth of bulk inorganic single crystals: melt, solution and vapor phase. 25 Single crystal growth from melt is the most common way used for single crystal growth via solidication and crystallization of the melted material. This method allows for the growth of large single crystals of excellent quality in a short period of time compared to other methods. 26 However, this method also has some limitations, such as difficulty in the maintenance of a stable temperature, gaining high melting points of some materials, and obtaining chemical consistency. Solution-growth technique is another approach where single crystals are obtained by dissolving the material into a suitable solvent or ux. 27 Among the solution-growth techniques, high-temperature solution growth or ux-growth is one of the most promising techniques for materials that incongruently melt, and can be applied for single crystal growth of the materials that cannot be processed by the melt growth process. The advantage of using this technique is that the single crystals of materials can be obtained below their melting temperatures. This is a slow process of crystal growth, and the existence of ux ions is inescapable in this method. Another method for single crystal growth is the vapor-phase growth via sublimation process at low temperature. This lower temperature enhances the crystal quality, avoiding impurities due to phase transitions. This method is used to obtain thin single crystal lms rather than obtaining bulk single crystals. However, this method has some limitations, such as low crystal growth and low vapor transport rates, for which this method is less utilized as compared to the other two methods. In spite of the three conventional methods mentioned above, there is one more technique known as solidstate single crystal growth (SSCG) that has received much attention recently. [28][29][30] This is a method via the solid-state conversion of polycrystalline materials into a single crystal.
The SSCG method overcomes all of the limitations found in the conventional methods. Control of the microstructure development during the conversion of polycrystalline materials into single crystals is the most challenging part of this method, for which it is limited to a few systems.
In this article, different MOFs and their composites for fuel production applications have been discussed based on previous reports.

CO 2 conversion
Global warming and the shortage of renewable energy resources are a major concern worldwide for the future energy supply and solving environmental issues. In view of the natural photosynthesis process where solar energy is converted into chemical energy, scientists are trying to develop alternate processes by means of the articial photosynthesis method where CO 2 reduction will produce hydrocarbons to fulll the energy requirements. Several efforts have been made to develop appropriate catalysts for CO 2 storage and conversion. Still, challenges remain for developing excellent materials for CO 2 storage and reduction into useful compounds and fuels, as not all types of catalysts are capable of giving the desired products. This leads to various difficulties of CO 2 reduction. These include high overpotential of CO 2 (À1.9 V vs. SHE), less selectivity for product formation, low reaction rate, and the poor solubility of CO 2 . 31 In addition, the band gap energy of semiconductor catalysts is another important factor for CO 2 reduction. For a spontaneous CO 2 reduction process, the conduction band position of the semiconductor should be higher or more negative than the redox potential for CO 2 reduction. Furthermore, the valence band position of the semiconductor should be lower or more positive than the redox potential for water oxidation into oxygen molecule, as shown in Fig. 3. Moreover, the reaction medium and reaction conditions such as temperature, pressure play important roles for CO 2 reduction. For efficient production of carbonaceous products from CO 2 , the chemisorption of CO 2 on the catalytically active sites of the catalyst surface is very important for activating the CO 2 molecule. To chemisorb CO 2 on the catalyst surface, proper dissolution of CO 2 in the solvent is another important factor. Since CO 2 shows poor solubility in the water medium, it leads to a low reaction rate of CO 2 reduction. To achieve the enhancement of CO 2 adsorption, researchers have found alternate ways such as the use of non-aqueous solvents (DMSO, acetonitrile, ethyl acetate, DMF) and a mixture of aqueous/non-aqueous solvents as the reaction medium, and the design of catalysts with large catalytically active sites. 5 It has been found that the use of sacricial materials such as triethanolamine (TEOA), triethyl amine (TEA), ethanol, and water can improve the catalytic activity for CO 2 conversion. CO 2 reduction leads to various useful chemicals and fuels like HCOOH, CH 3 COOH, CH 4 , C 2 H 6 , CH 3 OH, and C 2 H 5 OH, which are considered as energy carriers or fuels and have industrial applications. The multifunctionality and tunable functionality of MOFs make them promising catalysts for CO 2 storage and reduction activity.
There are various reports of CO 2 conversion into value-added chemicals and fuels using solar and electrical energy for MOFs-based materials. [32][33][34][35] Still, challenges remain for the largescale fuel production rate using MOFs-based materials for industrial applications. Fuels like methane, methanol, and acetic acid have tremendous usefulness in industry and as renewable energy resources. To limit the use of fossil fuels as energy sources, it would be benign to be used non-fossil fuels as energy sources to protect the energy for future generations. Methane is used as a fuel in home accessories such as ovens, water heaters, automobiles, and turbines. Rened liquid methane with the combination of liquid oxygen can be used as a rocket fuel. Methane gas is utilized to run or power engines and turbines in factories. Industries like food processors, pulp and paper, petroleum reneries and companies that work with clay, stone and glass utilize the energy it discharges. Methanol is used as non-fossil fuel energy sources in combustion engines of vehicles, or it can be mixed with gasoline to produce an efficient fuel like octane with lower emissions than the typically used gasoline. Methanol can also be used in industries, and as a fuel in cooking and thermal applications. Acetic acid is used in industries like food, chemical, pharmaceutical, chemical and plastic. It can also be used as a fuel to generate electricity via SOFC technology, which can be considered the most promising method for the generation of electricity for future power sustainability. Similarly, some more non-fossil fuels can be generated by the CO 2 conversion process, which have valuable uses in various aspects, including power generation.
Recent studies suggest three approaches for CO 2 reduction (a) photocatalytic approach, (b) electrocatalytic approach, and (c) photoelectrocatalytic approach. The photocatalytic CO 2 reduction is facilitated using solar energy and various powerful light sources like the Xe lamp and Hg lamp at particular wavelengths. The electrocatalytic process involves using electrical energy into useful products. The photoelectrocatalytic approach involves using both light energy and electrical energy sources. In the photoelectrochemical process, the use of light energy sources reduces the electricity consumption of the applied external voltage, and the application of an external voltage helps to separate the photogenerated electron-hole pairs, thereby enhancing the catalytic efficiency. Furthermore, the use of half cells inhibits the reoxidation of active nal products. All of these three approaches are very important to activate CO 2 reduction. Still, the photocatalytic approach seems to be an ideal way for CO 2 reduction due to high abundance of solar energy. Moreover, some drawbacks have been found for this process, such as the fast charge recombination rate and difficulty in generating a large number of catalytically active sites.
(a) Photocatalytic approach. MOFs with tailored hybrid inorganic-organic building blocks and large surface areas with uniform porosity enhance the CO 2 conversion rate. This is achieved by generating catalytically active sites on its surface and by transferring electrons from these catalytically active sites to activate the CO 2 molecule for the production of value-added chemicals and fuels. To activate the CO 2 molecule, the proper solubility of CO 2 in water is an important factor. Otherwise, it will become a major drawback for CO 2 conversion. There are some approaches to enhance CO 2 dissolution: the use of appropriate non-aqueous solutions, the suitable design of photocatalysts, the increase of the CO 2 partial pressure and having a high CO 2 ow rate. The photocatalytic CO 2 conversion depends mainly on three factors: (a) photoreactor design, (b) photocatalyst used, and (c) experimental parameters (temperature, pressure, solvent, light source, wavelength, catalyst concentration).
Lin et al. rst reported a doped UiO-67 metal-organic framework for photochemical CO 2 reduction using visible light by incorporation of Re I (CO) 3 (dcbpy)Cl(H 2 L 4 ) into the UiO-67 framework. 36 The photocatalytic CO 2 reduction activity into CO was studied using CO 2 -saturated with acetonitrile (MeCN) and triethylamine as a sacricial reducing agent (MeCN : -Triethylamine ¼ 20 : 1). The [Re I (CO) 3 (dcbpy)Cl]-doped UiO-67 showed excellent CO 2 reduction compared to the homogeneous catalyst, [Re I (CO) 3 (dcbpy)Cl]. The reduction process follows a unimolecular mechanism because of the uniform dispersion of the catalyst into MOFs-active surface sites [Re I (CO) 3 (dcbpy) Cl] (Re $50%) doping in MOF enhanced the photochemical CO 2 reduction activity.
Jiang et al. reported on one highly photoactive porphyrin based-Zirconium MOF catalyst, PCN-222 for the photocatalytic CO 2 reduction into a formate anion using solar energy. 37 Triethanolamine (TEOA) was used as a sacricial agent. Ultrafast spectroscopy with time-resolved photoluminescence spectroscopy unveiled that presence of a long-term electron trap state in the PCN-222 (Zr $2%) catalyst enhanced the photocatalytic conversion activity by inhibiting the electron-hole pair recombination and enhancing the reaction time. It provided greater activity to the PCN-222 catalyst than the H 2 TCPP ligand present therein. The photocatalytic activity was studied using 50 mg photocatalyst in MeCN/TEOA (10 : 1 v/v, 60 mL) solution, and the solution was irradiated using a Xe lamp (wavelength $420-800 nm). The spontaneous HCOO À anion production rate was found to increase by up to 30 mmol in 10 h, which was much higher as compared to previous reports of MIL-125-NH 2 and UiO-66-NH 2 under similar conditions. 38 This work conrms that the presence of an electron trap state in a catalyst can improve the catalytic efficiency (Fig. 4).
Choi and his co-workers reported on one nanoparticlecoated MOF photocatalyst for enhanced CO 2 reduction to CO. 39 The Re n -MOF photocatalyst contained two different functional units for the enhancement of the catalytic activity and catalytic stability, which were combined as a single construct. The highest photocatalytic activity was observed for Re 3 -MOF by controlling the density of the framework, where n ¼ 0, 1, 2, 3, 5, 11, 16, and 24 complexes per unit cell. They conrmed that the equilibrium vicinity between the photoactive centers enhances the photocatalytic activity. Based on this theory, they coated Re 3 -MOF (Re $23%) into Ag nanocubes (Ag 3Re 3 -MOF), which resulted in a seven-fold enhancement of the CO 2 conversion to CO under visible light irradiation. The photocatalytic experiments were studied using a sealed batch-type custom cell by dispersing the photocatalyst in a CO 2 saturated solution of acetonitrile/triethylamine, where triethylamine was used as a sacricial electron donor. The solution was irradiated using Xe lamp (300 W) with visible band-pass lters in a wavelength range of 400 to 700 nm.
As Cadmium sulde (CdS) has excellent visible-light responsive behavior and a suitably positioned conduction band for the photocatalytic reduction of CO 2 , there are still some limitations when using CdS as semiconducting photocatalysts due to the fast recombination of photogenerated electron-hole pairs, photo corrosion, lack of catalytic sites and low CO 2 absorption. 40 Recent progress has reported that CdS with MOFs combination can overcome these drawbacks by increasing the visible light response. [41][42][43] However, limitations arise for the CdS@MOFs composites for the efficient photocatalytic CO 2 reduction activity due to the lack of catalytic sites and the bigger size of CdS. 44 Han et al. reported on the ternary CdS/UiO-bpy/Co composites, where inorganic semiconductors and molecular redox catalysts were incorporated through UiObpy MOFs for the rst time to see the photocatalytic CO 2 conversion activity under visible light. 45 The ternary composites exhibited an excellent CO production rate, 235 mmol g À1 h À1 in 10 h irradiation, which was a 10.2-fold improvement in the CO production rate compared to the parent CdS (CO, 23 mmol g À1 h À1 ) and CdS/UiO-bpy (CO, 0 mmol g À1 h À1 ). The selectivity for CO was found to be 85%. The CdS/UiO-bpy/Co composites (Co loading 6.18 wt%) enhanced the separation and migration of the photo-induced charge carriers, thereby enhancing the CO 2 adsorption for better photocatalytic performance. The CO 2 reduction activity was studied using the photocatalyst in a mixer solution of acetonitrile and triethanolamine under visible light irradiation, where triethanolamine (TEOA) was used as an electron donor and acetonitrile as the solvent. The CO generation was increased with irradiation time. The CO evolution rate for CdS with Co-bpy as the co-catalyst was 110 mmol g À1 h À1 , which was lower as compared to the CdS/UiO-bpy/Co composites. Here, the CdS nanoparticle size was found to be smaller due to the presence of the bpydc bridging ligands.
Considering the potential use of carbon dots (CDs) that can act as both electron receptors and photosensitizers, Li et al. reported on CD-containing MOF photocatalysts for improving the CO 2 reduction activity. 46 They synthesized two types of photocatalysts for this study, one was CD decorated and the other one was the CD-embedded NH 2 -UiO-66 particles. During the study, they found that the positioning of CDs (28 mg mL À1 ) in MOFs greatly affects the photocatalytic activity of NH 2 -UiO-66 where CDs act as co-catalysts. The embedded CDs in NH 2 -UiO-66 exhibited better activity for CO 2 reduction than the CDdecorated NH 2 -UiO-66. From the charge kinetic investigations, they conrmed that the embedded CDs were more suitable for creating charge separation and promoting charge transfer in MOFs than the decorated CDs in MOFs. This is due to the possibility of a direct contact of the embedded CDs with the internal Zr-O clusters of MOFs, which builds up many small heterojunctions. In this work, CDs not only act as electron receptors for charge separation, but also act as photosensitizers in the photocatalyst, which enhance its photocatalytic activity for CO 2 reduction. They also conrmed that excessive CDs into MOFs can do harm to the photocatalytic activity. The photocatalytic activity was carried out using a Xe lamp (set at 100 mW cm À2 ) as the light source with a UV cut-off lter to achieve visible light at >420 nm. The catalyst was dispersed in a CO 2 saturated solution of CH 3 CN and triethanolamine, where TEOA was used as the sacricial agent. The CO production rate was 3.5 mmol gcat À1 h À1 with the pristine NH 2 -UiO-66 catalyst. For the CDs decorated MOF (CD/NH 2 -UiO-66), the rate was slightly increased to 4.0 mmol gcat À1 h À1 . The CO production for CDs embedded MOF (CD@NH 2 -UiO-66) was quite high at 16.6 mmol gcat À1 h À1 , which was 4.7 times greater than the pristine NH 2 -UiO-66 MOF. This conrms the importance of the 'location effect' of CDs in pristine MOFs for enhanced photocatalytic CO 2 conversion application (Fig. 5).
Maina et al. reported on one zeolitic imidazolate framework (ZIF-8) with controlled encapsulation of TiO 2 and Cu-TiO 2 nanoparticles using the rapid thermal deposition (RTD) method. 47 The photocatalytic performance of this composite towards CO 2 conversion was studied in the presence of UV irradiation for 6 h reaction cycle. The CO 2 conversion using a hybrid membrane reactor yielded methanol (CH 3 OH) and carbon monoxide (CO), and the yield was strongly dependent on the amount and conguration of the dopant semiconductor particles. The CuTiO 2 nanoparticles (7 mg)-doped ZIF-8 membranes exhibited the highest photocatalytic performance of the CO production yield, and was enhanced by 233% compared to the pristine ZIF-8 membrane. Furthermore, the methanol yield was enhanced by 70%. This work provides a new path to fabricate hybrid membranes encapsulating inorganic semiconducting nanoparticles and MOFs, with enhanced application in various elds. The photocatalytic efficiency of CO 2 conversion using hybrid membrane reactors was monitored taking both dimethylacetamide (DMAc) and acetonitrile (MeCN) as solvents. However, the photocatalyst exhibited the highest CO 2 conversion performance for methanol and carbon monoxide production using DMAc as a solvent as compared to MeCN. This is due to the high CO 2 solubilization capacity and stability of DMAc under UV light source. Furthermore, due to the higher Lewis's basicity of DMAc (0.73 B KT ) than MeCN (0.23 B KT ), DMAc exhibits more close interaction with CO 2 (which is a Lewis acid), thereby exhibiting favorable photo-conversion.
Sun et al. reported on the heterostructures of NH 2 -MIL-101(Fe)/g-C 3 N 4 (MCN-X) as photocatalysts with tunable surface structures, electrochemical, optical, and physicochemical properties to improve the photocatalytic CO 2 reduction rate via a solvent-free route. 48 Among these heterogeneous photocatalysts, NH 2 -MIL-101(Fe)/g-C 3 N 4 -30 wt% (MCN-3) exhibited an excellent CO 2 to CO conversion rate of 132.8 mmol g À1 , which was 3.6 times more than that of pristine NH 2 -MIL-101(Fe) and 6.9 times larger than that of sole g-C 3 N 4 . The NH 2 -MIL-101(Fe)/ g-C 3 N 4 composite with efficient interface electron transfer between NH 2 -MIL-101(Fe) (Fe $74%) and g-C 3 N 4 resulted in the improved photocatalytic CO 2 reduction upon visible light irradiation. This visible light-driven photoreduction of CO 2 by the MCN-X series was performed via a solvent-free route by loading the photocatalysts in the lter membrane, and employing TEOA as the sacricial agent. During the visible light driven photoreduction process, the heterostructure photocatalyst exhibited no HCOOH, CH 3 OH, H 2 and CH 4 generation as products due to the employed solvent-free process, which was conrmed by 1 H NMR spectral measurement and GC measurement. The only product formation was CO by MCN-3 with a higher photocatalytic efficiency rate of 132.8 mmol g À1 as compared to individual g-C 3 N 4 and NH 2 -MIL-101(Fe). MCN-4 with an increased amount of g-C 3 N 4 compared to MCN-4 reduced the catalytic activity, which may be attributed to the excess g-C 3 N 4 over the NH 2 -MIL-101(Fe) surface that blocks the interaction between CO 2 and the catalytic active sites. The proves the presence of a synergistic effect between the NH 2 -MIL-101(Fe) and g-C 3 N 4 . The GC-MS analysis ensures the formation of CO during the photoreduction of CO 2 (Fig. 6).
Chen and Duan et al. reported on the Au/PPF-3 composite, hybrids of thin porphyrin paddle-wheel frameworks-3 (PPF-3) nanosheets (PPF-3_1) anchored with AuNPs for the photocatalytic CO 2 conversion into HCOOH. 49 This helped to improve the morphology and assembly mode of PPF-3 between the AuNPs and PPF-3 nanosheets. Thin nanosheets in the Fig. 5 (a) Schematic illustration of two synthetic routes toward the CD-decorated NH 2 -UiO-66 particle and CD-embedded NH 2 -UiO-66 particle, respectively. The decorating strategy is based on different surface charges, and the encapsulating strategy is based on using CDs as seeds. (b) Electron transfer process and photocatalytic mechanism of two samples for CO 2 reduction. These figures have been adapted from ref. 46 with permission from the ROYAL SOCIETY OF CHEMISTRY, copyright 2020. photocatalytic process cause a quick charge transfer and high mass transport compared to thick nanosheets. Alternatively, nanosheets loaded with AuNPs will result in more effective light absorption than the AuNPs encapsulated nanosheets. The Au/ PPF-3 composite exhibits better CO 2 conversion in the MeCN/ EtOH system by plasmon resonance energy transfer process than pure PPF-3_1 or hybrids of thick PPF-3 nanosheets (PPF-3_2) supported AuNPs. PPF-3 nanosheets were accumulated with plasmonic AuNPs by electrostatic interaction to further enhance the light absorption ability of the Au/PPF-3 hybrids. The photocatalytic HCOOH production rate using Au/PPF-3_1A was $4-fold higher than sole pristine PPF-3_1 in the presence of visible light. The aspect ratio of the Au nanoparticles and PPF-3 MOF were 1 : 10 with 16% Co content in PPF-3 MOF for good CO 2 conversion activity. Through this work, they provided a potential and novel way to improve the rate of the photocatalytic activity by improving the MOFs framework via morphological assembly route.
Zhang et al. reported one Fe-porphyrin based-MOF, MAPbI 3 @PCN-221(Fe 0.2 ) encapsulating the Perovskite quantum dots, CH 3 NH 3 PbI 3 (MAPbI 3 ) for CO 2 photoreduction into CO and CH 4 . 50 The perovskite QDs in the MOF transfer the photogenerated electrons of QDs to the Fe catalytic site in the MOF to enhance the photocatalytic activity with 40% and 12% Zr and Fe contents in MOF, respectively. Using water as the source of an electron (sacricial reductant), the MAPbI 3 @PCN-221(Fe 0.2 ) photocatalyst exhibits a high photocatalytic CO 2 reduction yield of 1559 mmol g À1 to CO (34%) and CH 4 (66%), which is 38-fold larger than PCN-221(Fe 0.2 ) in the absence of perovskite QDs. Moreover, the MAPbI 3 QDs integration into MOF can signicantly enhance the stability of both LHP QDs and PCN-221(Fex). The photocatalytic CO 2 reduction experiments were performed in a CO 2 -saturated ethyl acetate solution containing a little water as the sacricial reductant and the solution was irradiated using 300 W Xe-lamp with a 400 nm lter, which yielded CO and CH 4 as the main reaction products for all PCN-221(Fex) and MAPbI 3 @PCN-221(Fex) photocatalysts containing Fe. In the absence of Fe in the system, CO was found as the only product for PCN-221 and MAPbI 3 @PCN-221 with little photocatalytic activity. For the photocatalytic systems, no H 2 and other liquid products like CH 3 OH and HCOOH were detected under similar irradiation time of 25 h. The presence of Fe plays a vital role for photocatalytic CO 2 reduction. MAPbI 3 @PCN-221(Fe 0.2 ) exhibited the highest yield for CO 2 conversion, leading to CO (104 mmol g À1 ) and CH 4 (325 mmol g À1 ) (Fig. 7).
Li and his co-workers reported on the UiO-66-NH 2 /graphene composite synthesized by in situ assembly of UiO-66-NH 2 onto graphene via microwave synthesis method for CO 2 conversion into formic acid and methane. 51 UiO-66-NH 2 crystals were highly dispersed on the graphene surface, and junctions between the graphene surface and UiO-66-NH 2 was formed. The UiO-66-NH 2 /2.0 graphene (2 wt% graphene) composite was capable of capturing CO 2 $73 cm 3 g À1 and exhibited high performance towards CO 2 conversion into formic acid (35.5 mmol) with good selectivity (78.6%) under visible-light irradiation. The high photocatalytic activity for the photoreduction of CO 2 with high selectivity was attributed to the small crystals size of MOFs, strong CO 2 sorption, well-dispersion of MOF on graphene, and strong junction between MOF and graphene. This leads to the generation of more charge carriers by trapping light and quick electron transfer from bulk to the MOF surface, increasing the stability of the system. The presence of amino groups in the MOFs structure enhances the CO 2 adsorption capability due to their basic nature. The introduction of graphene reduced the competitive reaction of hydrogen evolution (13.2 mmol) for the UiO-66-NH 2 /2.0 graphene composite as compared to pure UiO-66-NH 2 (16.9 mmol), and it led to efficient formic acid production during photocatalytic CO 2 conversion. Introducing graphene made the composite highly selective towards formic acid production compared to hydrogen evolution during the CO 2 photoreduction. The high selectivity of CO 2 reduction towards formic acid formation as compared to hydrogen evolution using the UiO-66-NH 2 /graphene composite can be attributed to the higher CO 2 adsorption and highly negative conduction band (CB) potential of the composite (À0.82 eV) that stimulates the higher reduction of CO 2 into HCOOH. The photocatalytic CO 2 reduction activity was carried out using 300 W Xe lamp, l >410 nm as light source (irradiation time $4 h), and DMF was used as solvent. TEOA was used as a sacricial agent and H 2 O (3 mL) was used as a proton source for the photoreduction of CO 2 (Table 1).
(b) Electrocatalytic approach. Electrocatalytic CO 2 reduction is another important approach for the production of valuable chemicals and fuels. This approach is very helpful for CO 2 reduction considering the high reaction rates under ambient conditions, easy tuning of the overpotential value and voltagedependent products. Still, challenges lie in the architecture of the catalysts, which can produce good catalytic efficiency with low applied potential, high faradaic efficiency, long-term stability of the reaction, good solubility of CO 2 in the electrolyte solution, low temperature working condition, and the use of a nonaqueous electrolyte solution. The faradaic efficiency is an important key for the electrochemical CO 2 conversion into value-added fuels and chemicals. Some catalysts exhibit excellent efficiency towards selective product formation in CO 2 conversion. Catalysts executing CO 2 conversion with low overpotential, high current density and high faradaic efficiency values are considered as the best catalysts for this purpose. Despite having several good reports for electrocatalytic CO 2 conversion with good faradaic efficiency, scientists are facing various difficulties on developing excellent catalysts towards the selective product formation of CO 2 reduction. This is due to the high overpotential value owing to the symmetrical structure of CO 2 and higher oxidation state of carbon, low charge transfer kinetics or low current density, consistency of the electrodes for shorter periods, CO 2 concentration, reaction temperature and pressure, pH, solvent selection, and product selectivity, which  are the main reasons affecting the efficiency in the electrocatalytic CO 2 reduction limiting their practical uses to some extent. MOFs-based composites are considered as an effective heterogenous catalyst for the electrocatalytic conversion of CO 2 into value-added chemicals and fuels considering their unique properties. The heterogeneous electrochemical reduction of CO 2 occurs at the electrode-electrolyte interfaces following these steps: 31 (i) chemical adsorption of CO 2 on the catalyst surface, where the cathodic electrode acts as a catalyst.
(ii) Electron or proton migration for C-O bond breaking and C-H bond formation.
(iii) Product species rearrangement, followed by desorption from the surface of the electrode and diffusion into the electrolyte.
Schroder et al. reported on one MOF-based electrode as an electrocatalyst prepared by electro-synthesis of MFM-300(In) on indium foil to study electrochemical CO 2 reduction. 52 Since the efficient CO 2 electro-reduction over the MOFs materials is suppressed due to the poor interaction between the electrode surface and thermally synthesized MOFs, it results in a low faradaic efficiency of the particular product and low electrochemical stability of the catalyst. The synthesized electrocatalyst, MFM-300(In)-e/In electrode, exhibits improvement in the conductivity as compared to the MFM-300(In)/carbon-paper electrodes. The resultant electrocatalyst, MFM-300(In)-e/In, exhibited exceptional activities for the electro-reduction of CO 2 with a current density of 46.1 mA cm À2 at an applied potential of À2.15 V vs. Ag/Ag + . The electrocatalytic CO 2 reduction results in the formation of formic acid aer 2 h electrocatalysis with an exceptional faradaic efficiency of 99.1%. Indium and [NiS 4 ] sites at the linker are responsible for the CO 2 absorption activity. The facile preparation of the MFM-300(In)e/In electrode with excellent electrochemical stability affords a new pathway for the development of efficient electro-catalysts for CO 2 reduction. They have also reported that the electrosynthesized MOF integrates additional framework In 3+ sites as structural defects, which enhances the charge transfer capability and promotes the CO 2 activation to the radicals, maintaining the stability and excellent electrocatalytic activity of the catalyst. The catalysis mechanism was studied by density functional theory (DFT).
Liu et al. reported a novel bismuth-based organic framework (Bi-BTC-D MOF) for the electrochemical reduction of CO 2 . 53 The Bi-MOF obtained by the conventional hydrothermal synthesis method was found to be very stable and effective to ERCO 2 . The resultant Bi-BTC-D MOF (42% Bi in MOF) produced formate as a product with a Faraday efficiency (FE) of 95.5% at a potential of À0.86 V RHE and a current density of À11.2 mA cm À2 . The FE was found to be very effective even aer 12 h of spontaneous electrolysis without signicant reduction. The experimental analysis conrmed that the excellent catalytic performance was attributed to the morphology of the electrocatalyst. DFT results conrmed that the Bi sites of Bi-BTC-D MOF played a major role in the efficient HCOO À production at very low overpotential value. The BTC and DMF ligands in the Bi-BTC-D structure control the catalytic activity of the Bi atoms effectively. This work reveals the potential uses of the MOFs materials for the electroreduction of CO 2 into sustainable fuels.
Yaghi and Yang et al. reported on thin lms of nanosized MOFs as nanoscopic materials for the efficient and selective reduction of CO 2 to carbon monoxide in aqueous electrolytes. 54 They synthesized cobalt porphyrin MOF, Al 2 (OH) 2 TCPP-Co (TCPP-H 2 ¼ 4,4 0 ,4 00 ,4 000 -(porphyrin-5,10, 15, 20 tetrayl)tetrabenzoate) for CO production with 76% selectivity and stability up to 7 h with per-site turnover number (TON) of 1400. From in situ spectroelectrochemical analysis, they conrmed the cobalt oxidation state during the course of the reaction, where they found that the maximum catalytic centers in this MOF electrocatalyst are redox-accessible, where Co(II) is reduced to Co(I) during catalysis. They further conrmed 6.1 Â 10 16 Co-atoms per square centimeter loading on MOF for the signicant CO 2 reduction to CO. This study signies the development of MOFbased electrochemical CO 2 reduction methods, where the catalytic active site, thickness/loading and inorganic backbone were chosen rationally, and the resulting MOF was incorporated into a conductive backbone. Such type of system modularity results in different prospects to further enhance the performance and develop new ways in electrocatalysis (Fig. 8).
Albo and his co-workers reported on Cu-based metal-organic porous materials (MOPM) supported on gas diffusion electrodes (GDEs) to accelerate the electrocatalytic CO 2 conversion to alcohols. 55 The four different synthesized MOPM-GDEs were named as, (1)  )] n . Theses electrodes exhibited comparatively large surface areas, large accessibility to the Cu catalytic centers, and favorable electrocatalytic CO 2 reduction activity with efficient production of methanol and ethanol in the liquid phase. The electrocatalytic reduction activity was carried out in a lter-press electrochemical cell under ambient conditions. The faradaic efficiencies of the electrodes for CO 2 conversion were obtained as HKUST-1 (15.9%), CuAdeAce (1.2%), and CuZnDTA (9.9%) at current density of 10 mA cm À2 . Among the four electrodes, the HKUST-1-and CuZnDTA-based electrodes exhibited stability in the electrocatalytic performance for up to 17 and 12 h, respectively. This work provides an idea to design efficient electrocatalysts for CO 2 reduction, including paddlewheel motifs that exhibit square planar coordination geometry around the Cu(II) centers and create open metal sites to cause strong interaction with guest species over the porous framework.
Dong and Feng et al. reported on one effective bimetallic twodimensional conjugated metal-organic framework (2D c-MOF) named as PcCu-O 8 -Zn with copper-phthalocyanine (CuN 4 ) as ligand and zinc-bis (dihydroxy) complex (ZnO 4 ) as a linkage for the electrocatalytic CO 2 reduction reaction (CO 2 RR). 56 57 The synthesized Cu 2 O@Cu-MOF electrocatalyst (Cu $33%) showed a time-responsive nature and large specic surface area for the strong chemisorption of CO 2 on the catalytically active sites of the Cu 2 O surface of the MOF composite. The generated charge-transfer was derived from the Cu 2 O core, rather than the Cu-MOF. The Cu 2 O@Cu-MOF electrocatalyst yielded hydrocarbons with a high hydrocarbon faradaic efficiency of 79.4%. The faradaic efficiency of CH 4 was found to be quite high at 63.2% at À1.71 V vs. RHE. The multiple functionalities (like catalysis, absorption capability and activation) of the MOF obtained from the combined effect of Cu 2 O and Cu-MOF are responsible for the high electrocatalytic performance and selective reduction of CO 2 to CH 4 (Fig. 9).
Heterogeneous electrocatalysts have the ability to ease the deactivation (like aggregation and dimerization) of active catalysts. Thus, these catalysts can improve the catalytic lifetime, catalyst's solubility in any solvent, and controllability over surroundings of the catalyst's active sites for enhanced performance. Heterogeneous catalysts have the capability to keep away the reduction catalyst from the electrode, in which the oxidation half-reaction (e.g., water to O 2 ) occurs. Based on the principle of heterogeneous electrochemical conversion of CO 2 to fuels under the requirement of high ux conditions to aggregate large reactant accessible catalysts on conductive surfaces via molecular catalysis, Hod and his group reported on the electrophoretic deposition of thin lms of a perfectly chosen MOF material for electrocatalytic CO 2 reduction into useful chemicals. 58 This was an effective method for immobilizing the required catalyst quantity. They reported on one Fe-porphyrin incorporated MOF, where the functionalized Fe-porphyrins behaved as structurally and catalytically competent, and as redox-conductive linkers. This approach resulted in the highly effective surface coverage of electrochemically addressable catalytic sites ($10 15 sites per cm 2 ). The resulting MOF composite produced CO and H 2 as CO 2 reduction products with $100% faradaic efficiency. The well-dened porosity of the MOF enabled the reactant, solvent and electrolyte access to the Fig. 8 The MOF catalyst allows for the modulation of metal centers, molecular linkers, and functional groups at the molecular level (A). The organic building units, in the form of cobalt-metalated TCPP, are assembled into a 3D MOF, Al 2 (OH) 2 TCPP-Co with variable inorganic building blocks (B). Co, orange spheres; O, red spheres; C, black spheres; N, blue spheres; Al, light-blue octahedra; and pyrrole ring, blue. In this structure, each carboxylate from A is bound to the aluminum inorganic backbone. The MOF is integrated with a conductive substrate to achieve a functional CO 2 electrochemical reduction system (C). This figure has been adapted from ref. 54 with permission from the AMERICAN CHEMICAL SOCIETY, copyright 2015. surface of the catalytic sites. The metalloporphyrin in MOF helps in the delivery of reducing equivalents to catalytic sites, which are not in direct contact with the underlying electrode.
Quan et al. reported on oxide-derived Cu/carbon (OD Cu/C) catalysts by a facile carbonization of Cu-based MOF (HKUST-1). 59 The electrocatalyst exhibited good selectivity, good stability and high activity towards electrochemical CO 2 reduction to alcohol products. The resultant catalyst, OD Cu/C-1000, yielded a faradaic efficiency of 13.8-8.3% and 31.4-34.8% for methanol and ethanol at À0.5 to À0.7 V, respectively vs. RHE. The onset potential for C 2 H 5 OH production was very low, which was approx. À0.1 V (vs. RHE), corresponding to an overpotential value of 190 mV. The excellent enhancement in the selectivity and activity of the electrocatalyst was attributed to the synergistic effect between the highly dispersed copper and matrix of porous carbon. For a mechanistic study for the electrocatalytic CO 2 reduction on OD Cu/C electrode, in situ infrared spectrum was employed. Based on DFT, they proposed the possible reaction pathway for CO 2 electroreduction on the OD Cu/C-1000 electrode, as shown in Fig. 10 (Table 2).
(c) Photoelectrocatalytic approach. Photoelectrochemical (PEC) CO 2 reduction into fuels and useful chemicals is one of the most promising approaches. Yet, there are many challenges as few reports are found using MOF-based materials for photoelectrocatalytic CO 2 conversion. The PEC approach is an articial photosynthesis method to produce valuable hydrocarbons that mainly uses the p-type semiconductor as a photocathode to conduct CO 2 reduction. The PEC of CO 2 follows both photocatalytic and electrocatalytic strategies. Thus, it utilizes the light energy sources to reduce the electricity consumption as compared to the electrocatalytic approach. The PEC approach results in an efficient catalytic rate due to the application of an external bias voltage that helps to promote the separation of photogenerated charge-carriers, which is very important to achieve high catalytic efficiency. No complete studies on PEC CO 2 conversion mechanism have been made until now. One Htype PEC reactor has been introduced with a suitable ionexchange process in most of the cases. Also, PEC reactor congurations (like microuidic, continuous ow, and PV) have attained great importance in recent years for their high performance in this regard. 60 The p-type semiconductors such as Cu 2 O exhibit harmful photo-corrosion and chemical changes. In view of this, Xiong et al. 61 reported on the Cu 3 (BTC) 2 /Cu 2 O photocathode by coating Cu 3 (BTC) 2 MOF on the Cu 2 O photocathode, which has the ability to stop the photo-corrosion of Cu 2 O and develop active catalytic sites for CO 2 reduction. By ultrafast spectroscopy, it was found that the newly formed interface can effectively enhance the charge separation and charge transfer. The MOF coating can enhance the activity and durability of Cu 2 O for PEC CO 2 reduction. This work gives insight into the architecture of progressive hybrid photoelectrodes and the importance of  interfacial charge dynamics in photoelectrocatalytic CO 2 conversion. The Cu-MOF/Cu 2 O photocathode converted CO 2 to CO with a faradaic efficiency of 95% without using any sacricial agent, as shown in Fig. 11. In this work, photoelectrochemical (PEC) experiments were performed using a three electrode quartz photoelectrochemical cell under irradiation, combined with an electrochemical workstation and a gas chromatograph (GC). The analysis was done using CO 2 -saturated acetonitrile containing 0.1 M tetrabutylammonium hexauorophosphate as a supporting electrolyte, and a 300 W xenon lamp with a 420 nm long wave-pass cutoff lter (l >420 nm) was used as the light source. The applied external potential for CO production was about À1.77 and À1.97 V vs. Fc/Fc + , which leads to 95% FEs (Table 3).

Hydrogen production
Hydrogen is a clean energy carrier that has various useful applications as fuel in power generation, vehicles and industrial applications, such as manufacturing of nitrogenous fertilizers and many useful organic products. Hydrogen is considered as one of the most promising fuels to overcome energy limitations for future generation. Therefore, the efficient production of hydrogen chemically using low-cost, feasible reaction processes has become very challenging for boosting the hydrogen economy. Nowadays, the water splitting process is considered an efficient route for hydrogen production efficiently using solar energy and other energy sources, along with hydrogen generation by CO 2 conversion process as discussed above. The design of efficient catalysts for this purpose is another important factor. Recently, many MOFs-based catalysts have been developed for hydrogen production considering the unique structural features of MOFs. 79 Enhanced light absorption ability of MOFs accelerates the charge separation of photogenerated electron-hole pairs for efficient hydrogen evolution reactions. Recently, chemists have reported on many MOFs composites containing nanoparticles, g-C 3 N 4 , traditional inorganic semiconducting materials, complexes, COFs, metals, and other functional materials with enhanced H 2 production rate. [80][81][82][83][84][85][86] Many MOFs derivatives using MOFs as templates like oxides and suldes have been reported for excellent H 2 production. 87 Water splitting reactions can proceed via three main routes: (a) photocatalytic water splitting, (b) electrocatalytic water splitting, and (c) photoelectrocatalytic water splitting. All three approaches are very convenient to carry out water splitting reactions for efficient hydrogen production. Recently, the photoelectrocatalytic approach has received much attention of the scientists due to the recent development, and considering that there are very few reports using both photo-and electro-catalytic methods.
(a) Photocatalytic approach. Photocatalytic hydrogen production is a very promising approach, which can utilize solar energy for conversion into chemical energy. Water plays the important role for photocatalytic hydrogen production, owing to its important properties like most abundant resource on earth and pollution-free hydrogen generation with high caloric value of hydrogen. Moreover, photocatalytic water reduction leads to two half reactions, oxidation and reduction, where oxygen and hydrogen are produced on the surface of the photocatalyst, respectively. 88 Water splitting is a 4 e À process thermodynamically that requires a Gibbs free energy (DG) of 237 kJ mol À1 , and it corresponds to a potential value of 1.23 eV. 88,89 Therefore, the water splitting reaction uses more than 50% of solar energy, and the two half reactions acquire high energy to proceed. Hence, both thermodynamic and kinetic considerations are important for water splitting reactions. The photocatalytic H 2 -evolution efficiency mainly depends upon the nature and amount of the photocatalysts and the irradiation source. The photocatalytic water splitting follows three main steps: (a) irradiation of the semiconductor photocatalyst to generate electrons in the excited state of the photocatalysts (CB or LUMO), and holes are generated in the valence band (VB) or HOMO of the photocatalyst. (b) The photogenerated charge carriers get separated by migration to the catalyst's surface. (c) The excited electrons on the photocatalyst surface i.e., in LUMO of the photocatalyst react with water to produce hydrogen, and oxygen is produced in the holes of the photocatalyst (HOMO). Desorption of gases from the surface of the photocatalyst then occurs. For a feasible water splitting reaction, the LUMO or CB of the photocatalyst must be more negative than the redox potential of water reduction (0.00 V vs. NHE, pH ¼ 0), and the VB or HOMO of the photocatalyst must possess more positive potential as compared to the redox potential for water oxidation (1.23 V vs. NHE, pH ¼ 0) (Fig. 12).
From the literature, 90 it was found that photocatalytic water splitting (PWS) has three main approaches: Among these three approaches, POWS is challenging because of the low photocatalytic efficiency, costly H 2 /O 2 separation, and reverse reaction mixtures. PPWS requires costly sacricial agents, and is not a sustainable one although it is commonly used. Furthermore, the photocatalytic rate is quite high. Recently, PIWS became the most challenging one. This approach is helpful for generating the most valuable byproduct H 2 O 2 , and no difficulty is associated with the produced H 2 /O 2 separation. Furthermore, PIWS provides one H 2 from two H 2 O molecules (Fig. 13).
Since the discovery of the rst MOF for photocatalytic hydrogen production using visible light irradiation, various MOFs with structural diversity as semiconducting materials for hydrogen production have been developed. 91 As compared to the traditional inorganic semiconductors, semiconducting MOFs have some advantages for use in photocatalytic water splitting reactions. These include the MOF's high porosity, tunable functionality, availability of catalytically active sites, enhanced charge separation ability, and introducing highly light-responsive co-catalyst or functional materials into highly porous MOFs surface. Pristine MOFs have direct uses as catalysts for photocatalytic hydrogen evolution on UV light irradiation. Silva et al. 92 reported on the highly water stable UiO-66 MOF for photocatalytic hydrogen production under monochromatic light irradiation at a wavelength of 370 nm. The photocatalytic activity was carried out in an aqueous solution of methanol, where methanol was used as a sacricial electron To enhance the photocatalytic activities of a semiconductor, heterojunction construction is an effective route to ensure the charge carriers opposite migration by conduction-band (CB) and valence-band (VB) offsets. 93 The cocatalyst loading on the semiconducting photocatalyst enhances the photocatalytic H 2 evolution rate. Zhang et al. 94 reported on the highly stable and visible-light responsive MOF, NH 2 -UiO-66, by constructing an excellent heterojunction with g-C 3 N 4 and encapsulating CDs into the pores of NH 2 -UiO-66 with 38% Zr in MOF to form a ternary composite photocatalyst, CD@NH 2 -UiO-66/g-C 3 N 4 , as carbon nanodots (CDs) have high light harvesting capacity and excellent electron transfer ability. In this work, the NH 2 -UiO-66/ g-C 3 N 4 heterojunction was built up at rst, and the CDs were incorporated into the NH 2 -UiO-66 pores. The CDs were generated from encapsulated glucose in the uniform pores of NH 2 -UiO-66 MOF. The ternary composite, CD@NH 2 -UiO-66/g-C 3 N 4 exhibited a hydrogen production rate of 2.930 mmol h À1 g À1 under visible-light irradiation. The rate was quite high as compared to that of the bulk g-C 3 N 4 , NH 2 -UiO-66 and NH 2 -UiO-66/g-C 3 N 4 . The optimum concentration of CDs was 2.77 wt%. The CDs as cocatalysts increase the electron transport properties and cause efficient charge separation. Moreover, CDs in the MOF composite serve as electron transfer mediation to initiate charge separation, improving light absorption and extending the lifetime of photo-induced carriers. This work demonstrates that CDs encapsulation into the pores of MOFs signies one effective strategy to improve the activity of MOF-based materials (Fig. 14).
Yang et al. reported a dye-sensitized Pd/MOF catalyst for photocatalytic hydrogen production. 95 A nanosized Zr-MOF, UiO-66 (Zr $40%) was synthesized solvothermally and Pd was loaded on Zr-MOF by impregnation reduction. The photocatalytic hydrogen evolution study was performed using TEOA as a sacricial donor in the presence of visible-light (l $420 nm), and eosin Y (EY) was introduced as a photosensitizer. The Pd/MOF with Pd loading of 3% exhibited the maximum photocatalytic activity of 2.28 mmol h À1 g À1 . The Pd nanoparticles in the Zr-MOF provided an electronic outlet, and the dye extended the spectral absorption range. The visible light source was a 5 W light-emitting diode lamp of wavelength 420 nm for studying the photocatalytic activity, and the evolved hydrogen was measured by gas chromatography. The mechanism of hydrogen evolution is shown in Fig. 15.
Light-absorbing chromophores such as porphyrin motifs, metal complexes, and NH 2 -BDC can be incorporated into MOFs to enhance the hydrogen evolution rate in the photocatalytic water splitting process, as these chromophore groups in the MOF framework enhance the energy conversion process by facilitating charge separation and improving the light harvesting capacity. Wang et al. 96 reported on the Zr-based MOFs introducing light-harvesting Ir complexes. Pt nanoparticles were introduced into the pores of the MOFs as a co-catalyst, which enhanced the photocatalytic H 2 evolution activity. The resulting Pt@MOF (Pt/Ir $17.8) composite exhibited efficient photocatalytic hydrogen evolution under visible light, with TONs of 7000, which was very high as compared to homogeneous systems.
Hybrid materials of covalent organic frameworks (COFs) and MOFs have drawn great attention for photocatalytic H 2  production due to their ordered long-range geometry, excellent light absorption capacity, large surface area, and tunable band gaps. Lan and his group introduced a 2-D COF (TpPa-1-COF) with stable MOF (NH 2 -UiO-66). 97 The MOF was covalently anchored on the surface of COF to form an interesting hybrid of MOF/COF. The resulting porous hybrid material NH 2 -UiO 66/ TpPa-1-COF, synthesized in a 4 : 6 ratio exhibited the maximum photocatalytic H 2 evolution rate of 23.41 mmol g À1 h À1 with a TOF value of 402.36 h À1 . The rate was about 20 times greater than that of the parent TpPa-1-COF. The experimental studies and DFT measurement conrmed that COF has effective light absorption capability with similar band gaps in between MOF and COF. There was also an efficient charge separation over the covalent heterojunction interface in the MOF/COF hybrid, which greatly contributed to the reaction rate for H 2 evolution. The Zr content in the MOF was about 12.72 wt% and co-catalyst Pt loading in COF was 3 wt% for the resulting MOF/COF hybrid for efficient H 2 production. Naggar et al. 98 reported on two MOF structures containing single and binary central metal ions, such as nickel-benzene dicarboxylic acid (Ni-BDC) and nickel/copperbenzene tri-carboxylic acid (Ni/Cu-BTC). The two MOFs were then used for photocatalytic hydrogen evolution activity, and it was found that Ni-MOF (Ni $67% in MOF) exhibited hydrogen productivity of 200 mmol h À1 with 50% purity. The Ni/Cu-MOF exhibited hydrogen storage activity due to the involvement of two metal cations within MOF (Table 4).
(b) Electrocatalytic approach. There are several industrial methods for hydrogen production, such as steam reforming, coal gasication, and water splitting. 99 Among these, the hydrogen evolution reaction (HER) by water splitting is gaining more importance due to advantages like performing the reaction at RT condition and at ambient pressure, easy separation of the reaction products selectively, and most efficiently, water is the most abundant component on earth. The electrocatalytic water splitting approach for H 2 production using electrocatalysts was found to be fruitful because of the capability of the electrocatalysts to lower the electric voltage utilization, yielding a high reaction rate. Electrolysis of water mainly involves two half reactions, the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), where HER occurs at the cathode and OER at the anode of an electroanalyser. 100,101 The electrochemical HER via water splitting process involves two steps: absorption of the formed hydrogen atom during the release of H 2 O or H 3 O + at the catalytically active sites (Volmer reaction) and H 2 formation and evolution. The details of the HER mechanism by electrocatalytic water splitting process using electrodes under different conditions has been discussed broadly in some previous reports. [102][103][104][105][106] The electrocatalytic hydrogen evolution reaction mechanism in both acidic and basic media has also been explained in detail in previous reports. [107][108][109] Knowing the complete mechanistic insights in both acidic and basic media of electrocatalytic HER would help to differentiate the reaction pathways followed by the electrocatalytic system. Efficient hydrogen production via electrolytic water splitting using MOF-based catalysts has become another important strategy to develop renewable fuels as an energy source. Pristine MOFs and MOFs-based materials incorporating a catalytically active moiety into its porous framework can serve as excellent catalysts for electrocatalytic HER (Fig. 16).
Carbon electrocatalysts comprising metal complexes (e.g., MN x and MS x ) are considered as promising materials to be used as replacements to costly Pt catalysts for HER. Seifert and Feng et al. 110 reported on carbon-rich 2D MOF-based catalysts incorporating molecular metal dithiolenediamine (MS 2 N 2 , M ¼ Co and Ni), metal bis(dithiolene) (MS 4 ), and metal bis(diamine) (MN 4 ) complexes, which were used as electrocatalysts for electrocatalytic hydrogen evolution. The 2D MOF-based catalysts with various metal complexes follow the order, MS 2 N 2 > MN 4 > MS 4 . The protonation happened on the metal atoms, which was conrmed from experimental and DFT calculation. The simultaneous H 2 evolution seemed to happen on the M À N units of the MS 2 N 2 active centers. The electrocatalytic HER performance of the 2D MOF sheets was performed using the rotating disk electrode (RDE) method in the N 2 -saturated aqueous solution. The three different modied electrodes of the 2D MOF-sheets (THTA-Co, THTA-Ni, and THT-Co nanosheets) and a bare electrode were taken in a solution of 0.5 M H 2 SO 4 for electrocatalytic HER, and it was found that THTA-Co-2D MOF sheets with CoS 2 N 2 active sites exhibited the best H 2 evolution performance as compared to the other three. Under a similar current density of 10 mA cm À2 , THTA-Co-2D MOF exhibited a lower overpotential value ($92 mV) and lower Tafel slope of 71 mV decade À1 . At equilibrium potential, the active site (CoS 2 N 2 ) of THTA-Co-2D MOF exhibited a lower Gibb's free energy of À0.12 eV than the other MOFs electrodes. The lower Gibb's free energy suggests weaker H* adsorption on the active surface sites and weaker adsorption of H*, leading to excellent catalytic performance for the HER route. The high exchange rate of the current density with the small overpotential value and low Gibb's free energy results in enhanced HER. The order of activity of THTA-Co-2D MOF was found to be CoS 2 N 2 (À0.12 eV) > CoN 4 (À0.4 eV) > CoS 4 (À0.42 eV) for the three active sites.
Loh et al. 111 reported on the GO/copper-MOF hybrid structure as an electrocatalyst for electrocatalytic HER, along with the oxygen evolution and reduction reaction. The catalyst has the ability to coordinate with more electronegative ligands of nitrogen and oxygen functional groups, thereby enhancing the stability of the framework structure, specically encapsulating GO in acidic medium. Graphene oxide (GO) in the MOF framework serves as linkers to MOF nodes, and also behaves as an electron transfer mediator. The GO-encapsulated Cu-MOF composite with 41% Cu in MOF provides a power density, which is 76% that of the commercial Pt catalyst. The (GO 8 wt%)/Cu-MOF hybrid exhibited the best photocatalytic performance for HER with smaller Tafel slope of 84 mV dec À1 with a minimum overpotential of À0.209 V vs. RHE at a current density of À30 mA cm À2 as compared to that of Cu-MOF and (GO 6 wt%)/Cu-MOF.
Duan and his co-workers reported on an ultrathin nanosheet array of MOFs on various substrates via dissolution-crystallization mechanism. 112 These materials showed excellent  performance for HER due to the presence of active molecular metal sites with ultra-small thin nanosheets, enhanced conductivity and ordered porosity. They designed a nickel-ironbased MOF array (NiFe-MOF), which exhibited a minimum overpotential value of 134 mV at a current density of 10 mA cm À2 and Tafel slope of 256 mV dec À1 for electrocatalytic HER. The presence of 23% Fe impurities in MOF enhanced the activities for Ni-based catalysts by introducing excess structural vacancies. In addition, they demonstrated excellent electrocatalytic performance for OER and overall water splitting. Roy and his co-workers reported a new MOF comprising cobaloximes that served as metallo-linkers between hexanuclear zirconium clusters. 113 While the reported MOF material, UU-100(Co), was grown on conducting substrates and a reduction potential was applied, the cobaloxime linkers seemed to accelerate electron transport over the thin lm. The TON was found to be much higher than the other equivalent cobaloxime system. The UU-100(Co) electrocatalyst with Zr : Co $2.8 AE 0.3 showed excellent electrocatalytic H 2 production from water at acidic medium. The Tafel analysis of the LSV data produced a Tafel slope of 250 mV dec À1 . When the catalyst was grown on glassy carbon, it exhibited electrochemical HER for 18 h at a constant current density of 1.7 mA cm À2 . Postelectrolysis analysis conrmed the intact molecular integrity of the cobaloxime linkers in the MOF material (Table 5).
(c) Photoelectrocatalytic approach. Photoelectrocatalytic (PEC) water splitting into hydrogen production is a very genuine eco-friendly approach that utilizes solar energy to produce oxygen and hydrogen molecules using a photocatalyst. The PEC water splitting mechanism proceeds through the following steps: (i) water splitting occurs at the photoanode of the electrochemical cell in the presence of solar light, creating photogenerated charge carriers. (ii) At the photoanode, oxygen is produced in the holes created. (iii) External circuits produce electrons at the anode, and these electrons get transferred to the cathode. (iv) H + produced at the anode moves to the cathode through the electrolyte solution to produce H 2 gas. The net reaction for H 2 production in PEC via water splitting is as shown below (Fig. 17). 114,115 2hv For a feasible PEC water splitting reaction, the light energy must possess energy equal to or more than the bandgap of the semiconductor photocatalyst. The rst PEC water splitting concept was introduced by Fujishima and Honda in 1972. 116 Since then, scientists have been trying to develop semiconducting photocatalysts for hydrogen generation by the PEC water splitting approach to convert photon energy of sunlight into electrochemical energy. MOFs with different structural diversity and semiconducting behavior, including the ability to introduce various functional materials into its highly porous surfaces, are drawing much scientic attention in recent years for the PEC hydrogen evolution reaction.
The PEC water splitting activity depends on various factors, such as the bandgap of the semiconductor photocatalyst, sample morphology, and other operating conditions. For efficient catalytic activity, efficient charge separation is a key factor. For efficient charge separation, the band gap of the photocatalyst should be smaller for absorbing a signicant amount of  solar energy (approx. 1.6-2.2 eV). 117,118 The particle size also changes the reaction rates. An optimum particle size is very essential for enhanced PEC activity, as the smallest particle size may lead to an enhanced electron-hole pair recombination rate with the close vicinity of charges, decreasing the reaction rates. 119 Lee et al. 120 reported on the photoactive aminefunctionalized Ti-MOF layer (MIL(125)-NH 2 (Ti)), which was uniformly coated on vertically ordered TiO 2 nanorods (NRs) through the hydrothermal process, and the photoelectrochemical (PEC) water splitting performance of the heterojunction photoanode was studied. MIL(125)-NH 2 /TiO 2 NRs exhibited a photocurrent density of 1.63 mA cm À2 at 1.23 V vs. RHE under AM 1.5 G simulated sunlight illumination. The photocurrent density was $2.7 times greater than that of pristine TiO 2 NRs. The water oxidation was enhanced by the efficient photon/electron conversion rate of the MIL(125)-NH 2 /TiO 2 NRs catalyst at a wavelength of l max ¼ 340 nm via efficient light sorption and charge separation. The MIL(125)-NH 2 coating enhanced the PEC activity in the TiO 2 NRs, which has been explained in detail by Lee and his workers in this work. Similarly, Hu et al. 121 reported on the Pt@NH 2 -MIL-125(Ti) catalyst for photoelectrochemical hydrogen production by a double solvent method, which was followed by photoreduction. The synthesized photocatalyst was used as a photocathode material for PEC H 2 evolution reaction. The 5 wt% Pt loading on MOF increased the photoelectrocatalytic H 2 production rate. Huang and Wu et al. 122 reported on the p-n heterojunction photoanode based on a p-type porphyrin MOF thin lm and an n-type rutile titanium nanorod array for photoelectrochemical water splitting. The TiO 2 @MOF core-shell nanorod array was synthesized by MOF coating on the TiO 2 nanorod array scaffold by a layer-bylayer self-assembly method. The formed p-n junction between TiO 2 and MOF improves the extraction of the photogenerated electrons and holes out of the TiO 2 nanorods. In addition, the MOF coating enhances the efficiency of the charge injection at the photoanode/electrolyte interface. Furthermore, the Co(III) introduction into the MOF layer improves the charge extraction capacity in the photoanode, thereby enhancing the charge injection efficiency. The resultant photoanode possessed a photocurrent density of 2.93 mA cm À2 at 1.23 V vs. RHE, which is $2.7 times the photocurrent achieved with a bare TiO 2 nanorod array under the irradiation of an unltered 300 W Xe lamp with an output power density of 100 mW cm À2 . Huo and Liu et al. 123 reported a review on MOFs as photosensitizers for photoelectrochemical water splitting reactions.

Conclusions and future aspects
Considering the serious global energy demand and extinction of fossil fuels due to daily human activities, it is highly advisable to discover and develop diverse new ways and models to cover up the annihilation gap for a sustained future generation with enhanced fuels and useful chemicals production. In this review, we discussed the recent developments on fuel energy production using MOFs-based materials. We have focused on different fuels production such as hydrocarbons (CO, CH 3 OH, CH 4 , and HCOOH) from green-house gases like CO 2 and H 2 production mainly from water splitting reactions, where the employment of different approaches like photocatalytic, electrocatalytic and photo electrocatalytic have been studied. Since the late 1990's until now, MOFs have gained much attentions of the researchers in a non-stoppable way owing to their unique structural properties. The use of MOFs in energy applications with efficient results are growing in the fastest way in recent years. Still, the production of highly efficient fuel sources from MOFs-based materials is restricted to the laboratory scale. Therefore, a fuel production rate in the industrial scale is highly desired. Moreover, CO 2 reduction into useful materials only in water medium leads to a low conversion rate due to the poor solubility of CO 2 in water. Researchers have developed a new strategy to overcome this problem using non-aqueous solutions or a mixture of aqueous/non-aqueous solvent, which are costly and not highly abundant. If water can be used as an effective solvent with good catalytic efficiency of the CO 2 conversion rate to develop standard catalysts, that will be highly appreciable to obtain efficient fuels production. Additionally, CO 2 conversion into efficient fuels having C 3 and C 3+ products are highly challenging these days. Very few C 3 products from CO 2 conversion have been reported until now. PEC CO 2 conversion into fuels like methanol and methane also has been a very challenging approach until now. In addition, the design of excellent MOFsbased catalysts to acquire the efficient production of fuels selectively is highly demanded to boost the world economy. In recent years, many MOFs materials have been utilized for photocatalytic, electrocatalytic and photoelectrocatalytic H 2 production from water splitting. Still, challenges remain in the design of perfect catalysts with high catalytic efficiency rate, and a proper mechanistic investigation has yet to be covered. The use of different functional materials such as co-catalysts into MOFs enhances the catalytic efficiency of the products formed. Also, sacricial agents improve the fuel production rate via water splitting and CO 2 conversion. Therefore, the development of MOFs catalysts to produce useful chemicals and fuels with high reaction rates without the assistance of any costly sacricial materials would be highly considerable. The interactive study of the functional materials with a porous MOFs surface is still not very clear, which requires the attention of the young researchers. Still, so many MOFs and MOFs-based materials with enhanced fuel production rate and selectivity have been explored so far. The unique structural properties and synergistic effect with other functional materials of highly semiconducting MOFs display tremendous activity in various elds, including energy conversion applications and fruitful fuels generation, providing renewable energy resources. Still, there are some limitations for practical applications of MOFs-based materials for fuels and chemicals production via CO 2 conversion and water splitting processes to date. Limitations include the appropriate catalysts and photoreactor design, low conducting properties, low conversion efficiency, lack of chemical stability, electrode potential, large band gap, insufficient catalytically active sites, suitable solvent selection, use of costly sacricial agents and the large-scale production of fuels and chemicals.
For an efficient energy conversion process using MOF-based catalysts, the catalysts should have appropriate band positions that can be achieved by tuning the MOFs structural functionalities or introducing highly light-responsive guest molecules into MOFs. Solvent selection is another important factor. Proper dissolution of catalysts or reactants into the selected solvent leads to efficient energy conversion processes. The catalysts surface should also possess higher catalytically active sites to achieve high catalytic production rates, which can be obtained by structural/surface modications of MOFs-based materials. For efficient electrochemical energy conversion, the electrode potential and current densities are two very crucial factors. MOFs-based catalysts also lead to selective chemicals and fuels production, depending on these factors. Most of the known catalysts have typical current densities on the order of mA cm À2 of the electrodes. However, in industries, the catalyst surface of the water electrolyzers operates at a magnitude of 2 A cm À2 . Experimental parameters (such as the temperature, pressure, catalysts concentration, wavelength and duration of illumination, dissolved oxygen concentration) are also important factors for the selective product formation during energy conversion reactions. Suitable co-catalysts loading on MOFs-based catalysts can enhance the catalytic energy conversion rates by changing the energies of the charge transfer process. Additionally, it can enhance the continuous ow of photogenerated charge carriers on the catalysts surface, providing stability to the system.
Limitations also include the use of costly materials that restrict the large-scale production of important fuels and chemicals using MOFs-based catalysts. The costly materials included are mainly costly non-aqueous solvents, sacricial materials, costly metals, and light sources (Xe, Hg lamp). Synthesizing cost-effective MOFs is very challenging nowadays, and scientists are working on it. To produce cost-effective MOFs, catalyst design should be based on low-cost materials that would additionally exhibit efficient energy conversion. For that, one should try to avoid the use of costly metals and solvents, and using of the easily available natural resources (like water) as a solvent and low-cost reactant materials to produce efficient MOFs-based catalysts with enhanced energy conversion applications. The loading of metal-free semiconducting co-catalysts on the MOFs surface instead of using metals containing costly semiconductors can be an alternative approach for producing cost-effective highly efficient MOFs-based catalysts with reduced toxicity for energy applications. Recently, researchers have found many ways for efficient fuels and chemical production using MOFs-based catalysts. Still, the production of cost-effective MOFs with large scale energy conversions remains a challenge for the upcoming generation to explore in this area.
Briey, we are focused on the recent highly commanded energy applications of MOF-based materials for fuel and chemical production purpose from CO 2 conversion and sustainable clean hydrogen production from water. We have mentioned some previous reports related to this topic. We are also trying to show the challenges associated with enhancing the catalytic rate of fuel production, so as to bring this challenge to the focus of researchers. Undoubtedly, MOFs are excellent materials for creating renewable energy resources, but the challenge still remains with dealing with the limitations of the MOFs-based materials and bringing it into practical uses.

Conflicts of interest
There are no conicts to declare.